COFs and TCPs are well-known examples of semiconductor devices carrying electronic components mounted on a wiring board. A COF, or chip on film, contains semiconductor elements mounted/joined onto a flexible wiring board. A TCP, or tape carrier package, contains semiconductor elements continuously joined to a flexible wiring board. The COF and TCP are chiefly applied to semiconductor devices containing a LCD (liquid crystal driver) IC.
Recent demand for LCDs with increased numbers of outputs is rapidly pushing the flexible wiring board for LCD ICs to finer wiring pattern pitches. Currently, the COF is more suited to fine-pitched wiring pattern than the TCP. Thus the COF is the popular choice for packaging of LCD ICs.
Now, a conventional fabrication method of a COF will be described in reference to FIG. 9.
First, the fabrication method of a flexible wiring board 50 is shown. A metal layer with a barrier function is formed by sputtering on a polyimide base component 51. Then, copper foil is formed by metalizing (copper plating). A photoresist is applied onto the copper foil and then cured. Thereafter, the photoresist is exposed and developed to form a pattern which matches the desired wiring pattern. The copper foil, along with the metal layer with a barrier function, is etched in accordance with the photoresist pattern. Removing the photoresist completes the transfer of the desired pattern. The process provides a wiring pattern barrier layer 52 and a copper conductor layer 53. The entire surface of the conductor pattern is plated uniformly with tin 58 to a thickness of 0.4 to 0.6 μm, to complete the fabrication of wires 59. Further, to provide protection to the wires 59, those parts of the surface of the wires 59 which have no relevance in connecting to the semiconductor chip are covered with solder resist 57. This completes the fabrication of the flexible wiring board 50.
The completed flexible wiring board 50 is joined to a semiconductor chip having gold bumps (protruding electrodes) 54. The joints are formed of eutectic AuSn between the tin plating 58 and the gold bumps 54. The joining step is called inner lead bonding (ILB).
After the ILB, the gap between the semiconductor chip 55 and the flexible wiring board 50 is filled with an underfill (i.e., thermosetting) sealing resin 56 to provide protection to the semiconductor chip 55. The sealing resin 56 is then thermally cured.
Thereafter, a final test as to electrical characteristics is conducted to complete the fabrication of the COF.
Demand has been growing recently for semiconductor devices with even more outputs. Accordingly, voltage on the wires 59 is on the rise, while the pitch for the wiring pattern is falling. However, the conventional semiconductor device cannot accommodate such high-voltage, low-pitch wires 59 and suffers electromigration between the wires 59. When a DC voltage is applied across adjacent wires at high humidity, the metal from which the wires are made can ionize and dissolve in electrochemical reaction. The wire material thus may deposit and grow where no wiring was initially provided. This phenomenon is termed electromigration. With a high voltage across one of the wires 59, the potential difference from an adjacent one of the wires 59 increases, which makes the wires 59 more prone to electromigration. Another factor is the spacing of the wires 59. As the pitches narrow down, the electric field strength of a wire 59 on an adjacent wire 59 increases, which makes electromigration more likely.
Electromigration results in metal ions depositing also between wires 59. The deposition short-circuits the wires 59, possibly leading to an isolation breakdown. The semiconductor device thus loses reliability. Accordingly, the prevention of electromigration over a long term is key to ensuring the reliability of the semiconductor device.
Occurrence of electromigration may be limited by several methods. One of them is to provide moisture prevention means preventing moisture from reaching the spacings between wires so as to prevent high degrees of humidity. The moisture prevention means may be provided, for example, by imparting moisture resistance to routes through which moisture can seep to the wiring. The possible routes include the base component of the flexible wiring board, the solder resist, and the sealing resin. However, all these members need be organic polymer materials permeable to water. It is difficult to completely stop moisture from reaching the spacings. Another technique is to apply a moisture resistant film. Such a film however requires a lot of labor and cost with only insufficient results in the limitation of electromigration.
A second method is to reduce contamination with chloride and other halogen ions which hastens the wiring material dissolving into metal ions. Halogen ions are however found in the material itself. It is difficult to completely remove halogen ions and like ionic impurities.
A third method is to reduce the electric field strength across the wiring so as to slow down the rate of the wiring material dissolving into metal ions. Nevertheless, narrowing connection pitches between the semiconductor chip and the flexible wiring board on which the chip sits and increasing application voltage across the wiring are inevitable to achieve high density packaging and high functionality of the semiconductor device. This in turn inevitably increases electric field strength across the wiring pattern.
Thus, electromigration cannot be prevented from occurring in the COF semiconductor device without significant cost, with the falling wiring pitches and high voltage application across the wiring. This presents an obstacle in developing semiconductor devices with high functionality.
Meanwhile, Japanese published patent application 11-144527/1999 (Tokukaihei 11-144527; published on May 28, 1999) discloses a conductive paste attaching electronic components onto the substrate. To prevent electromigration of silver ions, the conductive paste is mixed with a silver ion binder which forms a complex with silver ions. However, the conductive paste is not applicable to semiconductor wiring and falls short of addressing electromigration in wires.